When the exterior of a tubular workpiece, or a workpiece with a central hole, must be accurately machined, such as turned down on center or finished, it is impossible to hold it in the jaws of a standard chuck of the type described in my numerous earlier patents. Instead this workpiece is fitted over a mandrel normally having a cylindrical outer surface whose diameter can be increased to radially outwardly engage the inner surfaces of the workpiece, thereby holding it tightly on center.
The original such mandrels have a plurality of segments that can be spread mechanically apart, normally by sliding them along tapered cores that can displace them radially. Such purely mechanical arrangements can occasionally deform a workpiece, in particular flattening it at the regions aligned with the axially extending joints of the mandrel.
Hence the hydraulic power mandrel was developed. This type of chuck has a body extending along and centered on an axis, and having a pair of axially spaced outwardly projecting ridges. A cylindrical steel sleeve or tube is engaged over this body, spaced from it and forming a compartment with it between the two ridges. At each of the ridges the sleeve is welded to the body to seal off the annular compartment which therefore can be pressurized to radially swell the mandrel.
Thus a workpiece having an inner diameter slightly greater than that of the mandrel can be slipped over it, whereupon the compartment is pressurized to swell the steel tube tightly radially against it. This effectively locks the workpiece to the mandrel, on center and in excellent surface contact with the entire cylindrical surface of the mandrel.
One disadvantage of such a system is that the steel sleeve is only limitedly radially elastic. Hence it must fit very snugly in the recess or bore of the workpiece if it is to work. Furthermore the weld seams between the ends of the sleeve and the body are under considerable axial stress, as the sleeve inherently shortens axially somewhat as it swells radially, and vice versa.
In order to eliminate the likelihood of failure at these weld seams, it has been suggested to replace them with O-ring joints. This does allow limited axial displacement of the sleeve and mandrel body, but makes them prone to leakage since the radially deformable sleeve will pull radially away from the ridges when the compartment is pressurized. Such leakage is particularly likely at the extremely high pressures which such an arrangement is used with.